Surgical resection is still considered as the primary option for the treatment of malignant tumours. For unresectable tumours, several interstitial techniques that achieve local tissue destruction have been developed, including radio frequency (RF) coagulation, cryosurgery, ethanol injection, interstitial laser therapy and microwaves. Among these techniques, RF coagulation or RF ablation has shown the greatest impact on recent experimental and clinical research.
Radio frequency ablation (RFA) is used as minimally invasive heat-based method to ablate tumours. It is used primarily for the treatment of liver cancers but may also be employed to ablate malignant tumours of kidney, lung, bone, etc. For RFA, radio frequency waves are emitted from a generator through an non-insulated part of one or more electrodes that are inserted in the target diseased tissue. Tissue destruction in the form of coagulation necrosis is then caused primarily as a result of resistive heating in the immediate surrounding tissue and secondarily by passive heat conduction. Resistive heating only occurs within a rim of tissue in direct contact with the electrode because resistive heating is inversely proportional to the square distance between electrode(s) and tissue. Beyond this rim, tissue is further heated as a result of passive conduction. However, the RF emission is readily terminated as a result of impedance rise resulting from tissue desiccation and carbonization.
The first experiments with RFA on liver tissue were performed with a single plain metal electrode. The ablation diameter was rather limited, up to 1.6 cm, due to rapid rise in electrical impedance with current shut-off. The limited ablation diameter was insufficient to enable tumour coagulation through RFA. In order to be useful for tumour coagulation, the range of tissue destruction should cover the entire tumour and an additional rim of 1 cm of adjacent healthy tissue as a safety margin to avoid local recurrence.
The first technical challenge in the development of RFA was to design an electrode that could increase the range of coagulation lesion. Modified single-shaft electrodes have been developed and tested since 1994. In general, for different trends in the development of electrodes can be distinguished: internally cooled electrodes, expandable electrodes that enlarge the electrode-tissue interface and the electrical field, wet electrodes with saline perfusion through the electrode into the tissue, and bipolar electrodes.
In the article “Radiofrequency ablation using a new type of internally cooled electrode with an adjustable active tip: An experimental study in ex vivo and in vivo porcine livers”, the authors Jihoon Cha, et al. disclose a single shaft internally cooled electrode whose exposed active tip is adjustable through a covering insulating sheet that is connected to an operator adjustable switch. The electrode with adjustable active tip length is pictured in FIG. 1 of the article of Cha et al. In vivo experiments showed that different ablation volumes could be induced by adjusting the length of the exposed active tip.
As will be explained in the following paragraphs, several problems exist with single shaft electrodes.
Firstly, the commercialised versions of the above single-shaft electrodes produce coagulations that are smaller, less predictable, less regular and less complete than assumed. This results in a high rate of up to 60% of local recurrences of the tumour due to incomplete coagulation. The use of overlapping coagulations for larger tumours is insecure with high recurrence rates as a result of persistence of skipped areas. Furthermore, tumours usually do not fit the predefined elliptical, spherical or discoid shape that is generally ablated by these single-shaft electrodes. A consequence thereof is that tumours are incompletely coagulated again resulting in high recurrence rates, or large volumes of healthy tissue surrounding the tumour are coagulated.
Secondly, apart from the wet electrodes, these electrodes have a complicated design, can be used only once, and consequently are very expensive, currently between 1000 Eur and 1500 Eur.
Thirdly, none of these electrodes serve to coagulate multiple tumours each having their own shape and size. In clinical practice however, a patient may have several tumours. As a consequence, it may be required to use more than one single-shaft electrode in the same patient to adequately treat all tumours simultaneously or sequentially.
Further, some of these electrodes like the bipolar-wet electrodes can produce very large lesions but the size and shape of the coagulated zone is unpredictable. Such excessive coagulations must be avoided to prevent damage to healthy organ tissue and noble structures in the vicinity of the tumour.
In summary, single-shaft electrodes for RFA are insufficiently reliable and safe. They are not adaptable to tumours of any size and shape in a reproducible way, cannot sufficiently safe healthy tissue and vital structures surrounding the tumour, and they are complex and expensive.
The most promising evolution in RFA is the introduction of multiple electrode devices since 2001. A multiple electrode RFA device implements the combined use of plural electrodes. Monopolar and bipolar multiple electrode RFA devices can be distinguished. In monopolar mode devices, the electric current flows from all electrodes have the same polarity towards a grounding pad. In bipolar mode devices, the electric current flows between two electrodes or a group of electrodes that have differing polarities. Further, the multiple electrode RF devices can be categorized as sequential mode, simultaneous mode, or switching mode operated. When sequentially operated, the second electrode is activated after completion of the session of the first electrode, etc. When simultaneously operated, all electrodes are active during the same time interval. In switching mode, subgroups of electrodes are activated in an alternating fashion using a switch box and controller. The following paragraphs give an overview of known multiple electrode RFA devices operated in switching mode, and their limitations.
The article “A device for radiofrequency assisted hepatic resection” from the authors D. Haemmerich, D. J. Schutt, J. A. Will, R. M. Triegel, J. G. Webster and D. M. Mahvi, describes an RFA device with 6 electrodes held in position by a Teflon guide. As is illustrated by FIG. 3, a controller (PC) and an electronic switch box activate pairs of adjacent electrodes in switching bipolar mode, 0.5 seconds per pair. As described in section II C., second paragraph, the RFA process is further monitored by the device of D. Haemmerich et al. through impedance control per electrode pair. As soon as the impedance between a pair of electrodes exceeds a certain threshold, the power supplied to this pair of electrodes is disrupted for 10 seconds.
Although the multiple electrode RFA device described in the article from D. Haemmerich et al. enables to heat large slices of tissue of even length simultaneously through the rapid switching mode, it still does not enable to reliably coagulate a tumour of given shape and size. The device is not adaptable to a variety of patterns as a result of which its usefulness for clinical practice is limited.
In another article entitled “Multipolar Radiofrequency Ablation: First Clinical Results”, the authors J. Tacke, A. Mahnken, A. Roggan and R. W. Gunther describe a device with 3 electrodes inserted and held in position by aid of a plastic triangle with standardized distance control. The electrodes are saline-cooled probes that are activated pair-wise, one pair after the other, in bipolar mode. The switchbox allows for 30 possible combinations wherein the electrode pairs are alternately activated during 2 seconds each. The device of J. Tacke et al. further implements impedance control: the RF activation frequency is proportional to the tissue impedance, and if the tissue impedance increases beyond a limiting value, the coagulation process is ended.
Just like D. Haemmerich, J. Tacke et al. have disclosed a device that maximizes the achievable lesion size through a bipolar activation of multiple electrodes in switching mode, but which is not adaptable to any size or shape of a given tumour. The shape of the coagulated volume is rather pre-designed or pre-fabricated. Any deviation from the pre-fabricated shape requires multiple sequential insertions of the electrodes in the same patient, or excessive collateral destruction of healthy tissue.
In yet another article from the authors D. Haemmerich, F. T. Lee, D. J. Schutt, L. A. Sampson, J. G. Webster, J. P. Fine and D. M. Mahvi, entitled “Large-Volume Radiofrequency Ablation of ex Vivo Bovine Liver with Multiple Cooled Cluster Electrodes”, a comparison is made between the sequential, simultaneous and switching mode of an RFA device with 3 cool-tip electrodes that are held in position using a Plexiglas rectangle plate. The electrodes have a fixed exposed electrode length of 2.5 cm and are operated in monopolar mode. The article demonstrates that the most uniform heating is achieved when implementing the switching mode. The device further also implements impedance feedback to turn off power for 15 seconds whenever the impedance increases to a certain extent above baseline levels.
Although D. Haemmerich et al. have demonstrated that the switching mode is advantageous in achieving a uniform heating and tissue coagulation, their device does not adapt to any size and geometry of a tumour in a controllable fashion, and their prototype device is certainly not adapted to coagulate multiple tumours in a single patient in a reliable way thereby avoiding excessive healthy tissue and organic material destruction.
International patent application WO 2004/082498 entitled “Surface Electrode Multiple Mode Operation” discloses an RFA system with a base (102) and a plurality of electrodes with adjustable penetration depth and active tip length. The insertion depth and active tip length of the electrodes are made adjustable through screwing the electrodes and sliding electrically insulating sleeves extending from the surface of the base along the electrodes. The electrodes are operated in a bipolar fashion or combinations of electrodes can be selectively placed in bipolar arrangement with each other. A bipolar configuration as such however does not guarantee accurate and predictable coagulation adaptable to any size of tumour.
European Patent Application EP 1 645 239 entitled “Cool-tip Combined Electrode Introducer” describes RFA through a system with central reference electrode and circular positioned electrodes with adjustable insertion depth and active tip length. The insertion depth is monitored and also the radio frequency ablation (RFA) process is real-time monitored through dedicated monitoring equipment like an ultrasound scanner (15) and data processor (16). EP 1 645 239 however also fails to teach how the central electrode and circular positioned electrodes have to be activated in order to enable accurate and predictable coagulation adaptable to any size of tumour.
It is an objective of the present invention to disclose a device and method for radio frequency ablation (RFA) that resolves the shortcomings of the above described devices. In particular, it is an objective to disclose an RFA device and method yielding RF coagulation of diseased tissue that is predictable and adaptable to any size or shape of tumour. It is a further objective to disclose an RFA device and method that is able to treat large, non-spherical tumours, able to treat tumours near structures that may not be destructed, and able to treat multiple tumours of different sizes and shapes in a single patient. It is an additional objective of the present invention to present an RFA device that is not complex and costly, and that can be configured in a foolproof and reproducible manner to treat one or more tumours of any size or shape.